Introduction:
SpoilerMany people, when looking at classes in D&D, consider them based on their role in combat. When it comes to rogues and other skill-based classes, this is a mistake, since the true role of the skill-monkey lies outside of combat. Remember how Bilbo killed Smaug in the surprise round? No, you don't, because he didn't. Bilbo's role versus Smaug was to reconnoiter him, scout out his weak points, and steal treasure from underneath his nose. Most of the fighting, he left to others.
No, a skillmonkey's real asset is the ability to avoid a fight. Remember, you get just as much experience for sneaking past a guard as you do for killing it, and you get just as much loot from the villain if he's still alive at the end as if he's dead. A truly successful encounter, from the sneak's point of view, is one where the other side doesn't even know that there was an encounter.
To this end, an optimized skillmonkey isn't built around d6s of Sneak Attack damage; as the name implies, e's built around the use of skills. What BAB is to a warrior, or spellcasting levels are to a caster, skill points are to a skillmonkey: Your first priority is to keep your ranks high. However, unlike BAB or spellcasting levels, if you fall behind in your skill investment, you can catch up again on later levels. This gives skill-based characters a great deal of flexibility in choosing class levels, unmatched by other character types. You can even afford a few levels of low-skill classes in your build, so long as you make up for them later.
Skills:
Spoiler
As mentioned, the skillmonkey's most valuable abilities are his skills. Here's a discussion of some of the more useful ones.
Balance: It's good to have five ranks in this, since without that, you're considered flat-footed while balancing. Beyond that, though, you'll probably have enough miscellaneous bonuses that you won't need more ranks.
Bluff: A powerful social skill, and relatively unlikely to be nerfed by houserules. If you can make people believe what you want them to believe, you can make them do what you want them to do. Best suited for bards, due to Charisma synergy, the Glibness spell, and other social-enhancing abilities.
Diplomacy: How useful this is depends on what houserules your DM has in place for it (and almost all DMs will have some houserules). By RAW, a second-level marshal/binder can make Asmodeus, lord of the Nine Hells, his best friend in single round of using this skill. So don't do that, because that's only fun for about three seconds, and then everyone has to roll up a new campaign. If you have some more reasonable rules for it, though, it might be worthwhile. Again, like Bluff, this is best used by a bard.
Disable Device: There are many ways, available to many different classes, for dealing with traps. Most of them, however, involve setting them off, which is sometimes a Very Bad Idea. Disable Device is for when you don't want the entire dungeon to know that you're getting past the impaling spikes, deadfalls, and giant rolling boulder.
Disguise: Best used in conjunction with Bluff. Impersonation can get you anywhere, if you impersonate the right person.
Forgery: Like Bluff, except for when you're communicating in writing, not face-to-face. If you ever need to deal with an entire army, this is how.
Hide: Like invisibility, only better. It's just a DC 20 Spot check to notice the presence of an invisible creature, and you can get your Hide check up much higher than that. Plus, there are plenty of magical means to negate invisibility, but very few to negate Hide.
Knowledge: Can be very useful, but the party spellcasters are probably already covering the Knowledge skills pretty well.
Listen: Every character with access to Listen or Spot should max out at least one of them, so as not to fall into ambushes. There are some situations where one is useful but not the other, so it's not a bad idea to max both.
Move Silently: Goes along with Hide. There are very few magical effects which can replace Move Silently at all: The cleric's Silence spell, for instance, silences more than just you, so you might give yourself away by the cessation of ambient noise.
Open Lock: Can be partially replaced by the Knock spell, but sometimes, you want to be able to open a large number of locks, which costs a lot in spell slots or wand charges. I would recommend investing enough to be able to hit a DC 40 in an antimagic field using improvised tools, but the DCs stop scaling after that.
Search: Use in conjunction with Disable Device to deal with traps. Also use to find hidden doors, or anything else others don't want you to find.
Sleight Of Hand: This used to be called "Pick Pockets" in earlier editions, and that's still its primary use. Most foes become a lot less menacing when suddenly, they don't have a weapon, or spell component pouch, or holy symbol, or magic items. Can also be used to do other things with your hands that you don't want others seeing. This will effectively be your primary "attack", or way of doing bad things to enemies.
Spot: See Listen. In addition to defeating stealth, this skill is also used against Disguise and Sleight of Hand.
Survival: Useful if you have the Track feat. Otherwise, skip it.
Tumble: Trained-only, so it's good to have one rank in it. But you probably won't need much more than that.
Use Magic Device: If you're basing your build around UMD, you should probably just save yourself a bunch of gold and play a caster instead. But it opens up a lot of options for enhancing your existing abilities, so shouldn't be skipped.
Use Rope:Very useful, but all the DCs are low. You probably don't need even a single point in it.
Autohypnosis (XPH): Can save you from some bad situations, if you max it. If you're not maxing it, though, you might as well just rely on your saving throws, just like everyone else.
Iajitsu Focus (OA): If Oriental Adventures is allowed in your game, and you can get Iajitsu Focus as a class skill (as, for instance, through Factotum or Exemplar), it can give you a lot of damage, which also works on objects. Many DMs disallow it, though, since it was never updated to 3.5.
Skill tricks (CScoun): Not precisely skills, but you use skill points to get them. A skill trick is like a mini-feat, that gives you some new capability, or allows you to use a skill in a different way. They're especially important if you take the Uncanny Trickster PrC, which both requires them as a prerequisite, and gives you bonus tricks.
Use Psionic Device (XPH): See UMD. However, in most campaign settings, magic is more common than psionics, and there will be a larger variety of spells than of powers, so UMD is more useful.
Ability scores:
Spoiler
Strength: This is your dump stat. Strength is used for three things: Attack rolls, damage rolls, and carrying capacity. For those times you do end up in combat, you'll probably be using Dex for your attack rolls instead of Str (through Weapon Finesse or ranged weapons), and any Str bonus to damage will probably be insignificant compared to your precision damage. Meanwhile, for carrying capacity, you'll eventually be using a Heward's Handy Haversack, and before you get one of those, a pack mule only costs 8 GP.
Dexterity: This is the traditional skillmonkey's primary score. Many of your most important skills are based on Dex, and it also helps keep you alive. Some class features can reduce the importance of Dex, however.
Constitution: Everyone loves Con, of course, but if anyone can afford to skimp a bit on it, it's you. The rogue's primary defense lies in avoiding being targeted in the first place.
Intelligence: This is the key score for Search and Disable Device. More importantly, it also impacts the number of skill points you get. For most rogue builds, Int should be your second-highest score. If you have any Factotum or Beguiler levels, it should be your highest.
Wisdom: Few of your skills are based on Wis, and unless you're a ninja, swordsage, or cloistered cleric, none of your class features are Wis-dependent, either. Still, it adds to Listen, Spot, and Will saves, so it's not a complete waste.
Charisma: How important Cha is depends on how much you focus on social skills: If you make heavy use of Bluff, Disguise, etc., it's great, but you can also make a perfectly viable sneak with Cha in the cellar. If you have any Marshal in your build, though, its importance surpasses Dex, due to its use in minor auras.
Races:
Spoiler
Elf: The basic High Elf has a dexterity bonus and automatically makes Search checks for secret doors, and that's about all it has going for it. The Grey Elf, from the Monster Manual, also gets an Int bonus, which makes it worth considering, but there are still usually better choices.
Gnome: They really ought to get a racial bonus to Disable Device and Open Lock, but don't. On the bright side, they're small, which offers several advantages for a skillmonkey (most notably, a +4 to Hide). See also the Whisper Gnome, below.
Goblin: Similar in many ways to halflings, goblins also get Darkvision, a full 30 foot speed, and a larger bonus to Move Silently. The drawback is a -2 to Charisma, and being despised in most civilized lands. I wouldn't recommend one in a game with a lot of social interaction, but in a straight dungeon crawl, go for it.
Halfling: Small size, a bonus to your most important ability score, a penalty to your least important, racial bonuses to some useful skills, and rogue as favored class. This is probably your best choice for a core-only game. The only drawback is their lack of low-light vision or darkvision, meaning they'll need to carry a light source in dark areas. See also the Strongheart Halfling, below.
Human: Humans are good for any character type. The bonus feat is very nice, of course, but they also get a bonus skill point every level, which is even better for us. They also qualify for the Able Learner feat, which opens up many options for a skillmonkey.
Pixie: The +4 LA hurts, but the hefty ability score bonuses, invisibility, and flight just might be enough to make up for it in some builds.
Brownie (web): Like pixies, a fey with 4 LA. They can't fly or turn invisible, but they have even better ability modifiers, tiny size, better SLAs, and a very nice version of Hide in Plain Sight.
Changeling (ECS): Their racial disguise ability is very useful for a socially-focused character, and they get some very nice rogue racial substitution levels. If Eberron material is allowed, the all-around best race for intrigue.
Dark template (ToM): Can be added to anything. Has a LA of 1, but unlike many templates, it's well worth it. Hide in Plain Sight would almost be enough to justify it, but it also gives huge bonuses to Hide and Move Silently, Low-Light and Darkvision, a speed increase, and some cold resistance. Do note that the version of Hide in Plain Sight granted by this template does not remove the need for concealment (though that may be an oversight; check with your DM).
Gloura (Underdark): An ECL of 9, which puts them out of reach of low-level campaigns, but most of that is fey RHD, which give good skill points, and they get full Bard casting. The 2 LA buys you nice bonuses to Cha and Dex, plus damage reduction, flight, and Unearthly Grace like a nymph, and if you're using level adjustment buyoff from Unearthed Arcana, you can eventually be rid of the entire LA.
Illumian (RoD): Illumians don't get a lot of love, but their power sigils can give them a bonus to a great many different skills. They also count as humans, giving them access to Able Learner (but not the normal human bonus feat and skill points).
Strongheart Halfling (FRCS): All the advantages of a regular halfling, but with an extra feat, too. 'Nuff said.
Whisper Gnome (RoS): The absolute best race for stealth. Small size, a large bonus to Move Silently and Hide (in addition to the size bonus), a racial bonus to Dex, Silence as a spell-like ability, low-light and darkvision (so no need for a light source), full speed, and rogue as a favored class. Their charisma penalty hurts for social skills, but otherwise, an excellent race.
The Smallest Giant Ever (ECS, XPH): A trick that can permanently and undispellably turn a giant Fine-sized, and shift ability scores heavily from Str to Dex. All giants have LA, but a mere +1 from Half-Giant is by far worth it. Of course, most DMs won't allow this, but if you're playing in a sufficiently high-cheese game, it's hard to imagine a better option.
Class abilities:
Spoiler
This section covers some common class abilities you'll often get for skill-focused characters, either incidentally, or from seeking them out.
Concealment: Several different class abilities grant you the concealment you need to be able to hide, or bypass the need for concealment. This gives you the option of walking right up to your mark, without worrying about whether there's anything to hide behind.
Evasion: If you're having to make a saving throw, something's probably gone wrong already. Still, things do go wrong sometimes, so evasion is a nice insurance policy. It doesn't particularly relate to skills, beyond the fact that skillful classes often get it for free. Don't go out of your way to get the Improved version, though, since it only makes a difference when you fail a save, and your Reflex saves should be extremely high.
Hide in Plain Sight: There are at least six different versions of this ability, each of which applies in slightly different situations. But all of them let you use your Hide skill when you otherwise wouldn't be able. Since Hide is one of the most powerful skills available, this is something you really want to get.
Precision damage: As mentioned above, your primary role is not combat. However, many skillful classes end up getting some form of precision damage (sneak attack, skirmish, or sudden strike) anyway, and many folks enjoy throwing large numbers of d6s. The easiest way to optimize for precision damage is to dip into many different classes which gain it at first level. For instance, a rogue 1/spellthief 1/psychic rogue 1/ninja 1/scout 1/assassin 1 would be dealing 6d6 points of extra damage at level 6. Also useful is some means of doing things other than damage with your attacks, such as the Spellthief's magic-stealing abilities, the Crippling Strike rogue ability, or the ambush feats from Complete Scoundrel.
Skill Mastery: At mid to high levels, you need to consider worst-case scenarios for your skill checks, since if you can fail, eventually you will, and the consequences for failure can be severe or even lethal. Taking 10 on a skill check increases the worst-case scenario result by 9 points, making it almost as worthwhile as +9 to your skills.
Trapfinding: Essential for filling your primary role, and fortunately very easy to get, with many skillful classes gaining it at first level. You're likely, in fact, to pick it up multiple times, so you may want to trade out the extras for alternate class features, if available.
Classes:
Spoiler
Base:
Bard: Probably the best base class for a socially-oriented build. All of the social skills are in-class, you have Cha synergy with your spellcasting, and you have a variety of spells and class features (like Glibness and Fascinate) to enhance and complement your skill usage. But you also get a broad enough skill list that you don't have to just take social skills.
Expert: You obviously wouldn't choose Expert over a standard class. But if you have to take an NPC class for some reason, Expert at least gives a decent number of skill points and a class skill list as good as you like. You might conceivably take Expert if there's some rarely-used skill you want to max, and it's not on the skill list of any allowed class, but I have a hard time thinking of a specific situation.
Racial hit dice: In general, racial hit dice will hurt most characters, but Outsider, Dragon, and Fey RHD are all reasonable for a thief, since they all give a good number of skill points (class skills vary). Most races of those types come with LA attached, though.
Ranger: If you want high skills and BAB both, this is the easiest way to get them. The skill list is so-so, though: You get the stealth and awareness skills, but none of the social or trap skills. A highlight of the class at high levels is the Camouflage and Hide in Plain Sight abilities, both (ex), which let you hide regardless of cover or who's watching, so long as you're in natural terrain.
A powerful variant from Unearthed Arcana gives up the combat style feats for a limited form of Wildshape, which opens up many options. Another variant from Dungeonscape trades the ranger's tracking abilities for Trapfinding and Disable Device.
Rogue: The standard Core skillmonkey, you get access to all of the most important skills and a nearly unrivaled 8 points per level. The dead level at 20 is a peculiar design flaw, but not relevant in most games, and at any rate you still get your skill points. Still, you'll likely want to dip at least one level in Shadowdancer or some other useful class.
An interesting variant on the class is the Wilderness Rogue, from Unearthed Arcana, which shifts the class skill list to a bit more like the ranger's, and adds a few special ability options. Notably, the wilderness rogue can get the ranger's version of Hide in Plain Sight four levels earlier than the ranger himself can.
Sorcerer/Wizard: Spells, of course, open up many options for a character, but sorcerer and wizard both get a fairly bland skill list, and a base of only two points per level. If you go this route, you'll want to go into Unseen Seer as soon as possible.
Beguiler (PHB2): This is the simplest option for combining skills with spellcasting. You get all of the important skills, 6 skill points per level, and trapfinding, in addition to full spellcasting. Your spells are all fairly similar (mostly illusions and enchantments), but you can pick freely between them, and they include many of the most useful out-of-combat spells, making Beguiler a very solid option for handling intrigue. If you go Beguiler, you'll probably want to stick with the class for all twenty levels, or at most a one-level dip in something else, since almost any multiclassing will mean sacrificing spells, skill points, or both.
Binder (ToM), Dragonfire Adept (DrM), Warlock (CArc): The unlimited-use nature of these classes' abilities may appeal to many of the same players as skillmonkey characters. Unfortunately, they each only have 2 skill points per level, and there's no good way to advance any of them with high skills. So unless you're using a homebrew or Gestalt rules, you should probably set these aside.
Cloistered Cleric (UnArc): This variant of the cleric gives up some combat ability in exchange for getting six skill points a level and other abilities. Unfortunately, aside from all Knowledge skills, it's still got the same skill list as a standard cleric. You can either pick up class skills from domains such as Trickery or Kobold, or dip a more conventional skillmonkey class and take the Able Learner feat.
Factotum (Dun): Perhaps the best skillmonkey class in any book. All skills (including the odd ones like Autohypnosis and Iajitsu Focus) are class skills, and once per day per skill, you can add your class level as a bonus to a skill check. At third level, you get your Int bonus to all Str and Dex based skill and ability checks (including initiative checks), in addition to Str or Dex. The eighth level ability, Cunning Surge, lets you take extra standard actions in combat, which I personally think is too powerful. Unlike many, I do not actually recommend the Font of Inspiration feat: It's only really valuable if you take it many times, which cuts into the other feats available, and you won't be burning all that much inspiration if you don't use Cunning Surge.
Incarnate (MoI): Like most primarily magic-based classes, the Incarnate only has two skill points per level, but they have a fairly flat power curve: Most of what a 20th-level incarnate can do, a first-level incarnate can do, too, just not all at once. And several of their soulmelds boost skills, so it's a good class to dip. Don't take more than one level, though, before switching to Umbral Disciple.
Marshal (MiniHB): A single-level dip will get you a minor aura, which you can use to add your Charisma mod to yours and your allies' skills. Motivate Dexterity is of most use for a standard thief build, but Motivate Charisma is also good for a socially-oriented character. You might take as many as three levels, to get you a major aura (which isn't actually as good as the minor ones) and another minor, but with only four skill points and a lousy skill list, there's not much reason to take it any further than that.
Ninja (CAdv): Much like a rogue. The Ninja's variety of precision damage, Sudden Strike, isn't quite as good as Sneak Attack, and they get two fewer skill points per level, but in return, they get a variety of class features useful for avoiding detection (invisibility and spell resistance versus scrying), a Wisdom-based bonus to AC, and a few other benefits.
Psion (XPH): Like spellcasting, psionics adds options. Like most spellcasters, they also only get two skill points per level, so you'll want to go into Slayer and/or Elocator as soon as possible.
Psychic Rogue (web): With six skill points, Trapfinding, a good skill list, and special abilities similar to the rogue's at high level, the psychic rogue has the skillmonkey basics covered. They don't get very much manifesting, but their class power list covers most of the useful powers for their role, and you're not giving up much to gain them. Having access to psionics also opens up several other options. A solid choice.
Scout (CAdv): A wilderness-themed character, the Scout gets a full 8 skill points per level. Their class skill list is good (note that Disable Device was added in errata), but they're notably missing any Charisma-based skills, including Use Magic Device. Their precision damage is based on mobility rather than sneakery, and they also have several of the ranger's special abilities (including Camouflage and Hide in Plain Sight).
Spellthief (CAdv): A very peculiar class. They have most of the skills of a rogue and Trapfinding, but most of the rest of their class features are based on magic, not on skill use. They can steal magic from others, and can eventually cast a little of their own. They're quite versatile if you have other casters in the party they can borrow spells from, or if you face spellcasting opponents often, but otherwise, there are probably better choices.
Swordsage (ToB): The Swordsage gets six skill points a level and enough good skills to spend them on (though they're missing several good ones, and don't have Trapfinding). The primary reason to take the class, though, is the maneuvers, melee abilities similar to spells which are mostly useful in combat. They're probably the best of the skillmonkey classes at fighting, and some of their maneuvers are useful out-of-combat, too.
Prestige:
Arcane Trickster: As a great Mon Calamari once said, "It's a trap!" (and you should have been able to notice that, seeing as you have Trapfinding). A rogue-based class that gives full Sneak Attack progression but only four skill points per level is like a wizard-based class that gives full familiar advancement but only half spellcasting. For a wizard/rogue hybrid, the class you're looking for is the Unseen Seer. The only situation I would recommend this class is if you can qualify as a spellthief, since they can actually do something useful with their Sneak Attack dice.
Assassin: Good skill set, reasonable number of points, special abilities that mesh well with sneakiness, and a smattering of spells, this is the best rogue prestige class you'll find in the core rules. You'll always want to have a back-up plan for your death attack, though, since the save DC doesn't end up being all that high. The alignment restriction is problematic, since most PCs are non-evil, but that can be changed easily enough with DM cooperation, as illustrated by the Avenger
Shadowdancer: The best non-evil prestige class in the core books. In a core-only game, a one-level dip is worthwhile for a rogue to pick up Hide in Plain Sight. Once you start adding splatbooks, though, there are enough other worthwhile feats to take that the prereqs start looking pretty onerous, and there are enough other ways to get Hide in Plain Sight that it's not worthwhile any more. The higher-level abilities mostly just give you what you already have, so even core-only, there's little reason to take more than one.
Chameleon (RoD): Unmatched versatility. If you're going to dip into this class, the logical jumping-off point is level 2, when you get the changeable bonus feat. This can, for instance, be used on Open Minded, to be trained in a different skill every day. You also get spellcasting, which can come from any class list, and eventually boosts to your ability scores. It only gives four skill points, though. Just as a note, the Adapatation section at the end specifically says that the class (and the Able Learner feat) should also be allowed for changelings, if you're using them, for the ultimate in day-to-day mutability.
Elocator (XPH): The class of choice for combining psionic and skillful base classes. You lose a couple of manifester levels, but the skills stay good. You also get a variety of mobility-related abilities, which may make Scout a better base class to work from than Rogue.
Exemplar (CAdv): The class for anyone who's really, really good at some skill. The number of skill points (8) and the class skill list (all of them) both can't be beat. The prerequisites are easy, though relatively high-level. The biggest reason to take this class is Skill Mastery at level 1, but you also get a fair number of other skill-related abilities. Skill Artistry gives you a +4 bonus to one or more skills, but it's a competence bonus, so isn't all that useful. And the Persuasive Performance ability at level 4 can be used either for horrible game-breaking, or for flavorful fun, depending on what skill you use it with.
Heir of Siberys (ECS): All class skills from any of your classes remain class skills for Heir of Siberys, but you only get 2 points per level. The real reason to take this class, though, is that at 2nd level, if you choose the Mark of Sentinel, you can get Mind Blank 1/day (or other spells from other marks, but those aren't as useful to a thief). The third and final level gives another use of it, but that's not really needed, since Mind Blank lasts all day.
[Illithid] Slayer (XPH): This class has a lot going for it, but what we're most interested in is the 6th-level ability, Cerebral Blind. Good stealth skills combined with the Darkstalker feat can make you effectively immune to nonmagical detection; Cerebral Blind covers the magical side. Unfortunately, it only gives four skill points per level, but you can't have everything. In the Expanded Psionic Handbook, this class is called the Illithid Slayer, and includes a prerequisite that you kill an illithid one-on-one, but for copyright reasons the version on the SRD removes illithids both from the requirements and the name.
Umbral Disciple (MoI): You don't strictly need a level of Incarnate to enter this class, but it makes it much more worthwhile. You gain some extra essentia to power soulmelds, while also gaining six skill points a level and a solid skill list. You also get assorted shadow-related abilities, the best of which is Embrace of Shadow, at 3rd, which gives you both concealment and Hide in Plain Sight.
Uncanny Trickster (CScoun): This class has two things going for it. On the one hand, it gives you bonus skill tricks, which are of course useful. More importantly, though, it lets you advance some other class's features while maintaining 8 skill points per level and a roguelike skill list. This provides an excellent way to incorporate non-skillful classes into a build.
Unseen Seer (CMage): This is what Arcane Trickster should have been. Like Arcane Trickster, Unseen Seer gives full spellcasting advancement, but the entry requirements are much more flexible, and it gives six skill points per level. You also get to add divination spells from other classes to your list. The only real drawback is that some key skills are missing from the class skill list, such as Open Locks and Disable Device.